Offset lithography (offset or litho for short) is the most popular printing process. It is a printing process, typically with neither raised ink-printing surface segments on a printing plate, as in letterpress or flexography, nor recessed ink-printing segments, called cells, as in rotogravure.
Litho inks typically are oil-based, contain no water, have no or a small percent of volatile organic solvents, and have a paste consistency. Typically, litho inks are applied sequentially, one color at a time, from successive ink print stations. Typically, there are no dryers between print stations, so each color ink is still wet on the substrate, sheetfed or webfed, when the next layer of ink is deposited nearby or on it. This over-printing part of the process is called wet-trapping. Typically, a multi-color litho press has 5–10 print stations.
Typically, the conventional inks on sheetfed litho presses are still not dry when the sheets land on a delivery pile of the press, located after the last print station and connected thereto by an inclined ramp with a conveying mechanism therein. This is to say, there may be no drying equipment on-press before the delivery pile. Conventional litho inks dry and cure by a polymerization process in the delivery pile and thereafter, catalyzed by absorption of oxygen from the air. In other words, the drying and curing of conventional litho ink on sheetfed litho presses is a time-related affair. In contrast, flexo and gravure printing inks are quite fluid, typically contain large amounts of water or volatile organic solvents (VOCs), and have drying equipment after each print station to get rid of these volatiles. Thus, typically, flexo and gravure inks dry-trap (i.e. adhere to a dry surface on the substrate) at each print station, and there is no post-curing of the ink. And thus, typically flexo and gravure inks have a rather low amount of a film-forming material, in contrast to conventional litho inks which have a rather high amount of such material.
Another major difference between litho and the other three major printing processes is that litho uniquely deposits the ink indirectly onto the substrate. That is, the litho press has a printing plate, which does not deposit the ink thereon directly onto the substrate. The printing plate is carried on the surface of a rotating printing plate-carrying cylinder. The printing plate on this cylinder deposits the ink onto a layered rubber and fabric composite called a blanket carried on a rotating cylinder called a blanket cylinder. The blanket then deposits the ink onto the substrate. Hence, the derivation of the word offset to briefly describe the litho process. In contrast, the raised surfaces of letterpress and flexo printing plates and the recessed surfaces of gravure printing plates deposit the ink thereon directly onto the substrate.
This indirect transfer of ink from the litho printing plate to the substrate creates the need for an additional piece of equipment mounted adjacent to the printing plate-carrying cylinder called the dampener. The dampener applies a water and wetting agent mixture to the surface of the printing plate. There is an imaged portion on the printing plate surface which repels the water mixture and permits the litho ink to be deposited only upon the imaged surface thereon. The non-imaged areas of the printing plate with the water mixture thereon repel the litho ink and keep it off the substrate where it is not desired. There is thus no need for a dampener on the other three print process presses respectively which have only raised or recessed areas which receive the printing ink.
Also, because the litho ink is of paste consistency, many more what are herein called pressure rollers are required to transport the litho ink from its reservoir, called the ink fountain, onto the printing plate in the thin ink film needed for fidelity printing on the substrate.
In brief, litho is a much more complex printing process, but its typical production of better print fidelity on a wide variety of substrates has won its strong popularity. The sheetfed litho process is shown in detail in the drawings and is more fully described in the detailed description portion of this application.
All print processes have some drawbacks. One purpose of this invention is to show the litho printer, especially the modest family-owned sheetfed printer, how to overcome a major limitation, expense, and difficulty of applying gloss on paper, paperboard sheets or a web, and thus extend the capability of his existing equipment at modest capital expenditure. Also, the present invention provides print buyers with an environmentally-friendly, recyclable product, another advantage over the prior art.
Because there are many more sheetfed litho printers and presses than all other combined, with more people earning their livings in these plants, our drawings and discussion focus on sheets of paper or paperboard being printed on sheetfed litho presses. However, as above indicated, some of the principles involved in upgrading them also apply to webfed litho printing presses, especially in respect to sealing the somewhat porous print surfaces of the substrate to provide a better glossy, water-based overprint topcoat.
Many print buyers prize gloss on printed paper or paperboard, be it overall or selectively over portions of the printing. The attainment of high gloss generally commands a price premium because typically it is more costly to produce and typically requires more sophisticated and expensive production equipment than the typical modest family-owned printer has and can afford. Another purpose of this invention is to enable that modest family-owned printer to achieve high gloss at low cost in one pass through preferably existing, rather modest printing equipment already in his plant and which may not have coater equipment. Also, as previously indicated, it is preferable that the resulting product be environmentally-friendly and recyclable.
High gloss is typically defined as 80 minimum in the range of 80–92 on a 60° Glossmeter, a gloss-measuring instrument commercially available. Heretofore, high gloss is achieved in a sheetfed litho printing plant in one of two ways. One way uses a multi-color sheetfed litho printing press with or without an in-line overprint coater station, and an off-line ultra-violet (UV) light-generating and curing coating machine. This requires two passes of the sheets through the production equipment, and is another cost burden. The other way uses a multi-color sheetfed litho printing press with two in-line (meaning physically mounted on the press after the last print station) overprint coaters, a very expensive, sophisticated machine. The first of these coaters applies a water-based primer over the conventional litho ink, and the second of which applies a UV light-cured glossy topcoat over the primer. The function of the primer is to keep the still wet conventional litho inks away from the UV light-cured glossy topcoat, for these conventional inks poison the UV light-cured glossy topcoat and render it useless. These two coaters can add ¼ to ½ million dollars to the cost of a press, and typically the two coaters make the press run slower with more waste.
Virtually no conventional litho ink on paper or paperboard has a 60° gloss over 30. Many print buyers view this as a deficiency. So for fifty years or more, print buyers wanting higher gloss, specified X colors ink and a gloss-increasing varnish to be applied on the last print station of the litho press. The varnish is essentially a clear, unpigmented oil-based litho ink. Typically, it raised gloss levels 10–20 points to 40–50. As print buyers began to specify higher gloss than could be achieved with conventional litho ink and varnish, litho press manufacturers began to add a coater retractably mounted over the blanket cylinder of the last print station which applied the coating onto the blanket of the blanket cylinder. The blanket then applies the coating on to the substrate. Then still later, they physically mounted a coater, called the tower coater, on the press, positioned after the last print station. Said tower coater applies the coating directly onto the sheets after the sheets have left the last print station. The coater mounted over the blanket cylinder of the last print station is called a blanket coater. Either coater applies a rather fluid water-based emulsion, called a water-based coating, comprised of about 35–40% film-forming solids, formulated to wet-trap (i.e. capable of adhering to a wet litho ink surface) upon properly formulated conventional litho inks on the substrate. To dry these water-based overprint coatings (whatever their function) at or near rated press speed, the litho press manufacturer added a horizontal extended conveyor, after the tower coater and before the ramp, with its conveyor leading to the delivery pile. They equipped this extra space with drying equipment. Typically, the extra hardware added about $150,000 to $250,000 to the price of the press.
The coatings applied by either of these coaters of the prior art had various functions. These include: (1) to produce gloss; and/or (2) to increase scuff resistance; and or (3) to reduce or eliminate spray powder so as to provide a smoother product; and/or (4) to reduce the time needed for sheets to go to the next operation after they leave the press delivery pile, by preventing smudging of the still not fully dried ink.
Because these on-press applied fluid overprint water-based coatings contained only about 35–40% film-forming solids, the drying equipment after either coater had to get rid of 60–65% water, a difficult task at press speed. If the water was not eliminated, the coating was still wet and tacky and would cause sheets to stick together in the delivery pile; a phenomenon called blocking in the trade. Typically, these 35–40% solids water-based fluid overprint coatings when dry had a gloss in the range of 50–70, depending on coating formulation and the holdout character of the print surface of the paper or paperboard. This is a significant improvement in gloss compared to the aforementioned overprint varnish, and along with other attendant benefits, justified to many the cost of the extra equipment needed to attain this gloss level.
It is important to note that the fluid 35–40% solids water-based overprint glossy coatings do not have the necessary rheology to go through the many nips of the aforementioned multiple pressure rollers extending from the fountain. These fluid coating materials disintegrate in the third or fourth nip of these pressure rollers and can cause significant press damage.
To have the requisite rheology to go through the pressure rollers leading from the fountain to the printing plate on the plate cylinder without disintegrating, a glossy water-based coating needs about a 65% minimum solids content, 70% or higher preferred, along with a paste consistency. Such a coating deposited upon a typical paper or paperboard substrate by a planar-type printing plate typically does not produce gloss over 70. However, it does have the advantage of being able to be dried with suitable drying equipment in the standard delivery ramp at economic speeds because only 25–35% water needs to be eliminated from the substrate surface. However, such a coating can save the cost of a separate coater and an extended delivery.
What about the high-gloss minded print buyer who wanted his finished product to have gloss of 80 or higher. Nothing in the water-based coating prior art satisfied this buyer. As mentioned before, printers initially satisfied this buyer by installing the aforementioned special off-line (i.e., off-press) coating machines equipped with ultra-violet light producing lamps to cure the clear, transparent overprint coatings based on UV-curable chemistry. Typically, these UV-cured coatings were virtually 100% solids, contained no water, no VOCs, and cured almost instantaneously with proper energy output from the UV lamps. Depending on the absorbency, or the holdout of the printed surface they were deposited on, they produced 60° gloss in the range of 80–92. This is the derivation of the 80–92 level as the gloss target for printers to attain. The more competitive minded UV-curable printers focused on buying equipment which would permit them to attain the 80–92 gloss level in one-pass rather than the two-passes required with the off-line UV-curing coating machine. As previously explained, the second pass was eliminated by buying a litho press with two in-line coaters, but the cost of doing so was still too steep for the modest, family-owned printer. He needed a still lower cost method and hardware to attain the 80–92 gloss range.
Also, as previously indicated, the use of UV-cured coatings undesirably produced a product which is not recyclable.